This invention relates generally to feed-forward systems used to reduce distortion products in electrical devices such as amplifiers.
The term "distortion" as used herein shall be understood to include any signals present in the output of the electrical device which were not present in the input such as hum, noise, harmonic distortion, intermodulation distortion, etc.
In shipboard or ground-based high frequency communication systems, the receiving and transmitting antennas are often located close to each other. The receivers must operate at maximum sensitivity while the transmitters are transmitting. The amplifiers in the transmitters generate distortion products which are radiated from the transmitting antennas with the high power signals. These distortion products can fill the high frequency band with interference and adversely affect operation of close-by and remote receivers.
Feed-forward cancelling systems are known for reducing the distortion products generated in amplifiers. In the conventional adaptive feed-forward cancelling system, for example, samples of the input and output signals of a device such as a quasi-linear amplifier are adjusted in amplitude and phase and subtractively combined in a first combiner to produce a sample of the distortion products present in the output from the device. This distortion sample is adjusted in amplitude and phase and fed forward to a second combiner where it is subtractively combined with the output from the device to cancel the remaining distortion products therein. Typically, the distortion sample is adjusted in amplitude and phase by passing it through an amplitude-and-phase modulator followed by a subsidiary amplifier. Uncontrolled component variations such as drift can necessitate changes in the amplitude and phase settings of the modulator. To maintain the cancellation, a control means such as a synchronous detector correlates a sample of the feed-forward system output with the distortion sample and adaptively adjusts the modulator to minimize the correlation result. If D denotes the power level of the distortion component and S denotes the power level of the fundamental component (input signal replica) in the output signal from the device, and if the first combiner rejects the fundamental component relative to the distortion component by a power ratio R.sub.1, the second combiner rejects the distortion component relative to the fundamental component by a power ratio of EQU R.sub.2 =D.sup.2 R.sub.1 /S.sup.2.
The performance of this conventional adaptive feed-forward system suffers from the following disadvantage so far as its application to high frequency communication systems is concerned. To achieve a given amount of cancellation in the transmitter amplifiers one is forced to cancel the distortion products over the entire frequency band, but the receiving sensitivity is only affected by the distortion products in the frequency band to which the receiver is presently tuned.